1. Field of the Invention
The present invention relates to a surface acoustic wave device. More particularly, it relates to a surface acoustic wave device which are used as a high frequency filter and the like.
2. Description of the Related Arts
A surface acoustic wave device is a solid state high frequency device which utilizes the resonance phenomenon of surface acoustic wave propagating on the solid surface. The surface acoustic wave device is compact and thermally stable and has long life and good phase characteristic. In addition, it interacts with electron or light relatively easily. Therefore, a surface acoustic wave filter, which is an example of the surface acoustic wave device, is used as a medium frequency filter of a television set, etc.
Conventionally, the surface acoustic wave device is produced by forming interdigital electrodes on piezoelectric single crystal such as LiNbO.sub.3 and LiTaO.sub.3. Recently, it is produced by forming the piezoelectric layer such as ZnO on a substrate such as glass by a spattering method, etc.
However, the piezoelectric layer such as ZnO formed on the glass is not suitable for use in a high frequency band more than 100 MHz since it is a polycrystal substance which usually has orientation and many losses due to scattering. Therefore, the piezoelectric single crystal layer such as ZnO is grown on a sapphire.
In the surface acoustic wave device, a useful frequency is determined by a phase velocity of the acoustic wave propagating on the solid surface and an distance between interdigitial electrodes. Shorter the distance between electrodes is, or larger the phase velocity is, then at higher frequency the device is used. In order to achieve the high frequency, attempts are made to grow a piezoelectric C-axis orientated layer such as ZnO on alumina polycrystal which is cheaper than sapphire and achieves a higher sound speed than glass, but the satisfactory result is not obtained. Conventionally, the distance between electrodes is at least 1.2 .mu.m because of fine processing technique, and the fine processing technique is complex and a yield is not good.
When the electrodes are formed with a distance of 1.2 .mu.m, 2 GHz is the upper limit of the high frequency even if the piezoelectric layer is formed on the substrate, such as sapphire, which enables relatively high sound speed. The surface acoustic wave device which has extremely high frequency is required.